Normal air cleaners comprise a filter with an ionizing sector in which the particles comprised in the air flowing through the air cleaner are ionized. i.e. receive an electrical charge. The particles then pass a particle collector unit where the charged particles interact with an electrical field produced by plates charged with an opposite electrical charge. In this manner the particles are driven towards the plates and collide with the plates and are collected.
Swedish Patent DE-B-401 327 relates to an electrostatic filter which comprises wireshaped conductors, so called corona wires, and plates with different electrical potentials. There are particles in the medium flowing through the filter which are charged and attracted and struck onto the plates which have the reverse polarity relative to the particles charged during the passage between the corona wires. An insulating plate is arranged between each plate in order to prevent spark-over between the electrically charged plates, caused amongst others by the build-up of a layer of the particles attracted to the plates. The biggest disadvantage with such a filter is that one must use many plates, both charged and insulating, in order to achieve a suitable total filter area.
Another significant disadvantage is that after a relatively short period of time the filter loses a considerable amount of efficiency and particle collecting capacity. By reason of this the filter must be cleaned at relatively short time intervals which often is a laborious and time-consuming job.
Another disadvantage which appears with the above type of filter with electrically charged plates is that when the particles move in the field between the plates a number of the particles already become discharged at a distance from the plates through spark-over and the particles then may possibly not stick onto the plates but instead follow the air flow through the filter arrangement and out into the surroundings again.
Another type of filter device is shown in SE,A,7114330-9 (372 180). In this a device for the separation of particles out of a gas flow is described in which a number of high voltage electrodes and collector elements for the charged particles are alternatively arranged one after the other in the direction of the gas flow. The collecting elements consist of a network of paper, possibly metallized, which is earthed. The paper in the network has a weak conductivity.
It has now become apparent that the paper used for this purpose has a large disadvantage in that it is hygroscopic because in the case that the paper absorbs moisture it changes its characteristics with respect to its conductivity.
From JP-61-164664(A) a filter is known comprising corona electrodes and opposing electrodes which are electrically conducting and connected to the opposing pole (earth) of a voltage source with respect to the connection of the corona wires. Between these "opposingly charged" electrodes "acceleration electrodes" made of electrically insulating material are arranged. The distance between the corona electrode and the "opposingly charged" (earthed) electrodes is shorter than the corresponding distance between the corona electrode and the acceleration electrodes made of the electrically insulating material.
In this field it is also known to use sheet aluminum in the particle collection plates. This has, however, the disadvantage that during use it oxidizes. This means that the surface of the plates becomes more and more electrically insulating with a reduced particle collecting effect as a result. During the cleaning/regenerating of these plates strongly basic cleaning solutions must be used in order to destroy the aluminum oxide. This is obviously a big disadvantage.
Amongst the advantages of electrostatic filters is their relatively good particle collection capacity, at least when they are new. The above described filters, however, show a plurality of disadvantages, amongst others the difficulty of cleaning the particle collecting part for reuse. These difficulties are attributable to the construction of said filters.
In this connection it should be mentioned that electrostatic filters of which the plates are made of metal or in which metal is comprised, require a coupling resistance between the plates and earth. This has shown to be a disadvantage on one hand from the function point of view--if one or more of the plates is short-circuited, then a large part of the filter's capacity is lost. Because of this the filter is usually divided into sections with respect to the plates. Furthermore this construction in itself is a disadvantage from a recycling point of view because a number of parts constructed of different materials must be taken apart and sorted, e.g. metal plates, resistance wires, condensors and plastic rails in addition to the casing and other associated components.
Further, in the applicant's own application SE,A,9303059 with the filing date Sep. 13, 1993 an electrostatic filter for the separation of particles is described which comprises a particle charging unit arranged upstream comprising on the one hand a corona wire for the charging of the particles, and on the other hand a particle collector unit arranged 20 downstream relative to this and which is at least partially earthed. The particle collector unit comprises a plurality of plates arranged in a casing, whereat the casing and the plates are arranged to have essentially the same electric potential in relation to the corona wire. This is achieved through the plates and the casing being earthed and by the material in the casing and the plates each comprising a non-hygroscopic material with high resistivity, corresponding essentially to the mass resistivity of a so called soft-earthed material.
Even if the last filter solves some of the above mentioned problems, it has surprisingly been shown that further improvements are achievable with a filter according to the invention of the type given below.